In other words: implementation of AttachmentResource.doSearch(..) (see as an example among many here with ProviderResource.)
Some basic query parameters will need to be supported such as:
This task will encompass adapting the current UI side code to leverage the new REST endpoint rather than ObsResource. See here in the gallery directive.
please study what the kind of requests made by the gallery directive to ensure that AttachmentResource.doSearch(..) will be backward compatible.